Various measures have been taken to store plants (e.g., fruits and vegetables) in a container used in marine transportation for a long period of time while keeping the plants fresh. As an example of such measures, a technique for keeping freshness of plants, while taking the fact into account that freshness of plants considerably decreases as they breathe, has been known. In this technique, gas having a lower oxygen concentration than the outside air is supplied into the container to decrease the oxygen concentration of the air in the container, thereby reducing the breathing of the plants and keeping the plants fresh (see, e.g., Patent Document 1 below).
Patent Document 1 discloses a gas supply device in which air is guided to an adsorption column filled with a nitrogen adsorbent, and is pressurized to have the nitrogen in the air adsorbed onto the adsorbent. After that, the adsorption column is depressurized to recover the nitrogen adsorbed on the adsorbent, thereby generating nitrogen-enriched air with a nitrogen concentration higher than the outside air. This nitrogen-enriched air is supplied into the container.